With a conventional steering system steerable wheels are steered by a driver via an actuation unit, more preferably through twisting a steering wheel. The rotating of the steering wheel causes a displacement of a rack which in turn pivots the wheels. In power steerings the steering movement of the driver is supported through an additional electric motor. Here, the steering support is generally performed through characteristic-controlled or regulated systems where the steering support is applied as a function of the vehicle speed.
In addition, steering systems are known where no mechanical connection exists between the steerable wheels and the steering wheel (steer by wire). Here, the wheels are pivoted as a function of the angle of rotation of the steering wheel with the help of appropriate motors.
To determine the position of the steering wheel and the steered wheels, rotary angle-of-rotation sensors are generally employed, in the steering train mostly on the steering column, on the steering pinion direct or on the input shaft connected with the latter. These can be designed to determine the absolute steering angle either over the entire steering angle range or only over 360°. In the latter case, the angle-of-rotation sensor can determine the absolute steering angle only in conjunction with a centre finding algorithm.
These steering angle sensors can be designed as both so-called multi-turn sensors, which measure the absolute steering angle over more than one turn from lock to lock or as single-turn sensors which can trace the absolute steering angle only over one steering revolution. In addition, incremental angle sensors are also known.
In an electric power steering the steering angle can also be determined via the position sensor of the electric motor which is directly or indirectly tied to the steering column with a certain ratio. Based on the ratio between steering column and electric motor, the angular signal of said position sensor which is absolute on its own is likewise not an absolute one relative to the steering column.
When using an angle sensor which merely measures absolutely over one steering wheel revolution, only an approximate position determination is required to determine the centre, while with incrementally acting angle sensors or measurement via the position sensor of the motor, far greater accuracy of the position determination is required. This can take place either via evaluation of the wheel rotational speeds or via evaluation of the steering moment and the steering angular velocity. In both cases however it requires a certain time or travelling distance in order to find a sufficiently accurate centre position by means of these centre finding algorithms, as is required depending on the utilisation purpose of the angular signals. Following a system restart this restricts the scope of performance of the comfort and safety systems dependent on the steering angle for a certain time which impairs their availability.
In addition, electric power steering systems generally utilise a separate torque sensor assigned to the steering system for measuring the steering moment applied to the steering wheel by the driver. This torque sensor mostly serves exclusively for measuring the torque, but can also be combined with an angle-of-rotation sensor.